1. Field of the Invention
The present invention relates to a laser diode and a manufacturing method of the same, and more particularly to a single-mode laser diode comprising a light waveguide including a multi-mode waveguide region and a manufacturing method of the same.
2. Description of the Related Art
Opto-electronics technologies have been utilized and developed in various fields such as the information input/output technologies typified by compact discs (hereinafter referred to as CDs) and the optical communication technologies using optical fibers, and so on. Laser diodes, (hereinafter referred to as LDs), have been developed as devices which support these opto-electronics technologies. For example, LDs of near infrared band or visible band are used for CDs and LDs of long wave band are used for optical communications. As described above, various kinds of LDs contribute to the opto-electronics technologies.
There are various structures of LDs. Among waveguide type LDs, the waveguide type LDs having a waveguide which obtains a so-called single-transverse-mode light are generally used. Concerning CDs, in order to increase the capacity of information, it is important to increase the recording density. And it is necessary to use single-transverse-mode light to increase the recording density. Moreover, with regard to optical communications, there is a problem that the multi-mode signal light is not suitable for long distance communication because of the influence of multi-mode dispersions. For this reason, a waveguide type LD which emits single-transverse-mode light is generally used in both the field of the information input/output and the field of the optical communications.
In order to obtain this single-transverse-mode light, from the waveguide of the LDs, a single-transverse-mode waveguide which has a narrow waveguide width to cut off the multi-mode light is generally used. More concretely, in the single-transverse-mode waveguide, the width of the active layer in the waveguide of the LD is limited in the range of about 2 to 4 μm. Therefore, an electric current capable of being injected into the LD is limited to a relatively small value so that there is a limitation to an output of light.
In order to allow a high injection electric current and to enhance a saturation light output level, one of the easiest ways is to widen the waveguide width of the LD. However, as described above, since there is the limitation that the waveguide width must be relatively narrow in order to realize the single-transverse-mode waveguide, a technological restriction for achieving a high output performance of the LDs exists.
In order to solve the foregoing problems, various kinds of methods have been proposed. A mode-filter integrated multi-mode LD is reported in IEEE Journal of Quantum Electronics Vol. QE-23 No. 6, 1987, pages 730-737, by Patrick Vanwikelberge et. al. ( hereinafter referred to as the first prior art). In this multi-mode LD, a main light excitation region is constituted by a multi-mode LD having a wide waveguide width, resulting in an increase in a saturation light output performance.
As another way, a flare-shaped LD is reported in Electronics Letters Vol. 32, No. 24, 1996, pages 2277-2279, by M. Sagawa et al. (hereinafter referred to as the second prior art). This LD has a structure in which a waveguide width in its one end and the waveguide width becomes broader toward its other end showing a flare shape. Since the waveguide width in its broader end is broader than conventional single-transverse-mode waveguides, the light output performance is increased. In addition, since the mode of the output light is controlled by the single-transverse-mode waveguide region at the narrow end, the foregoing LD can be constructed such that it can keep the single-transverse-mode light in spite of the broader width in its other end, by forming it to the desirable flare shape.
Moreover, as another way, a Phase-locked LD array is reported in Applied Physics Letters Vol. 60, No. 6, 1992, pages 668-670, by L. J. Mawst et. al. (hereinafter referred to as the third prior art). This LD array has a plurality of LDs, for example, 20 LDs, which are integrated by arranging them perpendicular to the direction of the its light waveguide at certain intervals and the LDs are allowed to resonate with each other, thereby finally achieving a high single-transverse-mode output.
In the first prior art, though the multi-mode waveguide region excites not only the single-transverse-mode light but also primary and secondary lights, the single-transverse-mode light can be obtained by removing the primary and secondary lights by mode filters. Since light energies of the primary and secondary lights do not contribute to the single-transverse-mode light output of the LD, there is a problem that an electric/light conversion efficiency is low compared to the conventional single-transverse-mode LD.
From the view point of manufacturing the LD, the second prior art involves a disadvantage of difficulty in forming a desirable flare shape. Specifically, in the case that the length of the LD involves a little error or the broader end of the waveguide is not formed exactly according to the design due to an error in manufacturing, the flare shape is no longer the most desirable one. In this case, the desired characteristics of the LD can not be obtained.
The third prior art has a complicated structure and involves a difficulty in manufacturing so that it is difficult to manufacture the LD array with a high yield. Moreover, in the third prior art, a structural tolerance to satisfy the resonance conditions is severe, that is, the allowable range of the error of the third prior art is narrow, so that it is difficult to manufacture the LD array with a high reproducibility.
As described above, the allowable injection electric current into the LD having a narrow waveguide width to obtain the single-transverse-mode light, which has been heretofore used in general, is limited to a small quantity, so that there is a problem of limitation of the output of the LD. The three prior arts that have been proposed in order to solve these problems involve the problems that it is difficult to obtain the high electric/light conversion efficiency, the LD can not be manufactured with a high reproducibility, the manufacturing tolerance is severe, and the structures are complicated.